Ok, MiFlash recognized my phone after sending the cmd via console.
I now tried to install the Fastboot ROM, but got the following error:
i[dot]imgur[dot]com/Lkdqt3J.jpg
Sorry for posting it like this.
But I can't post links yet and the image upload doesn't work for some reason.
Ok, I could imagine this is a tough one:
I had problems with my GPS, so I wanted to get root access.
Contacted Xiaomi to unlock my phone: worked fine.
Next up to get it rooted, I installed a fresh version of the TWRP recovery, since the old one for some reason disappeared after I installed the...